In projection systems, the light source used is usually one or more high-pressure gas discharge lamps (HID lamp or UHP [ultra-high-performance] lamp). These lamps can in principle be operated both with DC current and with AC current. Usually, however, operation with AC current is preferred since this prevents rapid erosion of the electrodes and increases the efficiency of the lamp.
However, particularly in the case of operation with an AC current, the risk of unstable arc discharges also increases on account of the polarity change, and these arc discharges may lead to flickering of the output light current. This is based essentially on the fact that the arc discharge is dependent on the temperature and the condition of the surface of the electrodes, and moreover the temporal courses of the electrode temperature differ in the phases in which the electrode acts as anode and as cathode. This in turn leads to the fact that the electrode temperature changes considerably during one period of the lamp current. In order to favorably influence these fluctuations, a (first) current pulse having the same polarity as the half-period to be changed is generated at the end of each half-period of the lamp current, i.e. prior to a change in polarity, and this current pulse is superposed on the lamp current so that the overall current increases and the electrode temperature rises. As a result, the stability of the arc discharge can be considerably improved.
These first current pulses lead to the lamp being operated with an AC lamp current which has more or less pronounced pulsed components which in turn give rise to a correspondingly pulsed increased light current.
Particularly in color projection displays which operate by means of time-sequential color display methods, fluctuations in the output light current may have a disruptive effect when one of the primary color images is displayed at a different brightness than the other primary color images.
In such a time-sequential color display method, the color image is produced on the display by the time-sequential displaying of at least three complete images in each case in one of the three primary colors (“field sequential color”) blue, green and red and possibly a fourth white image (white segment). This method is currently used for example in most DLP® (digital light processing) projectors (which operate with one color wheel) and in future may also be used in LCoS systems in which at present prisms and in future a drum-like arrangement are used for the time-sequential illumination of the display with the primary colors.
In order to prevent the increase in the light current which is brought about by the first current pulses from leading to a color shift in the displayed image, the current pulse generation and color modulator are usually synchronized with one another as explained in EP 1 154 652 in such a way that the first current pulses always occur in the fourth white image.
For operation of the lamp and the technical circuit design of the lamp driver, the absolute value of the amplitude of the first current pulse (and not the relative level thereof compared to the mean lamp current) is furthermore to be set and kept as constant as possible. Since, however, the operating voltage rises in many cases (or possibly also drops) during the service life of the lamp and thus the mean current in the lamp decreases, the ratio between the amplitude of the first current pulses and the mean current becomes increasingly large. This in turn means that the component of white light in the overall image becomes increasingly high and the color saturation thus becomes increasingly low. Although this effect could be compensated by measuring the current levels and calculating a correction, this would mean that the image brightness decreases quite considerably as the lamp becomes older. (The opposite effect occurs if in certain lamps the operating voltage drops over the course of the service life.)